[inputs.icinga2] differ between handled and unhandled host/service problems

## Feature Request

Opening a feature request kicks off a discussion.

### Proposal:

In the current version of the icinga inputs plugin, the current state of a host or service will be collected.
However, you cannot distinguish between handled and unhandled hosts/services.

### Current behavior:

You have only this host/service states:

> state - The state: UP/DOWN for hosts, OK/WARNING/CRITICAL/UNKNOWN for services

### Desired behavior:

A new tag where you can see if host/service problems are handled or not.
You already see it in Icingweb --> Overview --> Tactical Overview, but it would be nice if telegraf can collect these metrics as well.

### Use case:
I want to know how many hosts/services problems I have and how many of them are already handled.

Start with the inputs.icinga2 plugin and trace how current host and service states are collected. Determine how Icinga exposes whether a problem is handled, then define the additional tag or metric so handled and unhandled problems can be distinguished and verify the collected output against the requested use case.

Written by the indexing model from the issue text.